The generation of electricity through the use of electrical generators is important for modern life. These generators require some source of external energy to operate, which may for example be some type of fossil fuel and/or renewable energy. The generator then consumes the energy source in order to generate electricity. However, it is important that the power output remain constant in order for the generated electricity to be usable.
The power output can remain constant if the shaft speed of the generator remains constant. However, the shaft speed of the generator cannot always be held to a constant rate. Therefore, some generators have relied on maintaining at least a minimum speed, such that the power output provided is determined according to the minimum speed of the shaft. If the shaft speed increases beyond the minimum, the excess power produced is discarded and hence is wasted.
Various solutions have been attempted but none has completely solved the problem for alternating current (AC) generators. For example, U.S. Pat. No. 5,541,483 provides a method for controlling a direct current (DC) motor or generator, particularly those of the permanent magnet type. Due to functional differences between AC and DC generators, the described solution would not be operative for an AC generator.
US Patent Application No. 2004/0257050 describes a method and device for constant current generation, which attempts to overcome drawbacks associated with potentially variable shaft speed through controlling the current that is output by the generator, thereby achieving a constant level of output current. Therefore, the described invention relates to current stabilization which is relatively complex.
Various attempted solutions for achieving stabilized voltage output have included using inertia or friction as a mean of moderating the shaft speed fluctuations; using torque control in order to control the shaft speed; or converting the fluctuating AC electricity to DC electricity, and then converting it again into the standard grid AC power. Clearly all of these methods are very wasteful of energy.
In an effort to use sources of renewable energy, systems and devices have been introduced which use “natural” energy such as wind, through the use of wind turbines; sun, through the use of solar panels; water power, such as wave or tidal power; and the like. Wind energy may be particularly variable, given that wind tends to increase and decrease in power, and/or change direction, quite regularly. However, all of the “natural” energy sources may be expected to suffer from instability of power levels. Thus, for renewable energy, the ability to convert non-stable mechanical power to stable electrical power has major demand in a variety of applications.
Various solutions have been proposed in this area to overcome the instability of renewable energy sources, particularly with regard to wind generation. For example, U.S. Pat. No. 7,068,015 provides a solution for wind power by adjusting the magnetic field according to the rotation speed of the wind turbine according to feedback determined by measuring output voltage or current. US Patent Application No. 2004/0119292 provides a method for controlling the shaft speed of the generator by controlling its torque, a solution which is disadvantageous as noted above. The taught method further requires a diode rectifier for operation, which is another disadvantage.
U.S. Pat. No. 5,083,039 controls the power output by controlling the magnetic field of the generator, by controlling the stator current. However, changes to the stator current cause changes to the generator torque. In order to compensate for changes to the torque, the shaft speed is controlled by changing the pitch of the “wings” or blades of the turbine, which may be disadvantageous due to wind conditions, and which is disadvantageous in any case because it requires an additional expenditure of energy. U.S. Pat. No. 6,137,187 is similarly disadvantageous as it requires a pitch control system.